The primary structure of 333 amino acids from mouse and rat LDH-B4 (heart) are being determined by sequencing both protein and cDNA. A comparison between mouse and human LDH-B sequence revealed eight (2.4%) amino acid differences: four differences are clustered within the random-coil region of amino-terminal 20 residues, two substitutions at residue numbers 52 and 132 are located in the beta-sheet, and two changes at residue numbers 236 and 317 are positioned in alpha-helix.